1. Field of Invention
The field of the invention is a tin oxidation inhibitor for an electrolytic tin halogen plating composition and a process for coating metallic substrates, such as an iron-containing substrate, employing the composition.
2. Description of Related Art
Electrolytic tin halogen plating compositions are employed for the continuous or semi-continuous electrolytic deposition of tin coatings on a steel strip. The composition is employed in an electrolytic cell and the strip passed through the cell. Stannous tin (Sn (II)) salts in the halide plating bath can be oxidized to stannic tin (Sn (IV)). The large surface of a strip line presents a large area of solution which will be available for air oxidation. The common 140.degree. F. operating temperature enhances the activity of the solution and the loss of stannous tin by oxidation to stannic tin. Other oxidizing agents in the plating cell also account for this oxidation. Stannic tin forms metastannic acid, an insoluble tin compound that precipitates and forms sludge in the plating cell. As a result the plating process must be stopped periodically and the plating cell cleaned. The consequent lost production time translates into lost profits as does the loss of stannous tin.
Producers of tin can stock employ the halogen plating solution in large volumes. Production is oftentimes a continuous or round-the-clock operation performed on large strip plating machines and consumes tons of tin metal.
Halogen tin baths contain large amounts of chloride and fluoride ion in solution. These aggressive ions corrode the moving sheet steel before it can be coated with the inert tin, especially where only one side of the steel is plated during the first half of the plating cycle. This results in the very harmful, but unavoidable introduction of ferrous iron ion (Fe (II)) into the plating solution where the ferrous ion has a natural tendency to oxidize to the ferric ion (Fe(III)) by reacting with the air present at the large surface area. Iron in either oxidation state harms the bath.
Large amounts of ferrous iron can co-deposit with the tin. The resultant alloy will not reflow at low temperatures nor provide a corrosion resistant surface, which is essential for tin plated steel.
Producers know the importance of keeping ferric iron out of the bath because it reacts with stannous tin, oxidizing it to stannic tin while being reduced to ferrous iron. Ferric iron is the main cause of loss of stannous tin and the resultant production of metastannic acid sludge.
Introducing highly soluble sodium ferrocyanide into the plating solution provides ferrocyanide ions that react with the ferric iron and forms an insoluble blue material commonly known as Prussian Blue (ferric ferrocyanide). This removes ferric iron from the bath precipitating as a sludge at the bottom of the tank.
Mixing metastannic acid in the precipitate with the Prussian Blue creates not only a larger volume of waste, but also raises environmental concern because of the cyanide content in the sludge. It would therefore be an advantage to minimize or eliminate ferrocyanide materials from the bath.
As it is not feasible to totally eliminate the admittance of iron into the solution, it would be an advantage to remove the iron before conversion to the ferric form or prevent the formation of ferric ion by providing a reducing environment in the solution. The present invention provides this reducing environment.
The invention comprises a composition and process for treating a stannous tin (Sn(II)) halide plating bath to minimize, substantially minimize, or prevent the oxidation of the stannous tin to stannic tin (Sn(IV)).
Salm, U.S. Pat. No. 4,508,480, describes a composition and a process for producing tin plate by electrodeposition of a halogen-tin composition onto a continuous steel strip. The process includes steps of treating the steel strip by electrolytic cleaning, light pickling, electrolytic tinning, thermal reflowing of the deposited tin and a final chemical or electrochemical "passivation" treatment.
Thermal reflowing, also known as "flow-brightening," involves melting the plated tin coating by conduction, radiation or high frequency induction heating to a temperature slightly above the melting point of tin whereby tin flows to produce a smooth bright surface and a portion of the tin combines with the steel of the base strip to form an alloy layer.
Halogen-type electrolytic tinning involves a series of small cells which contain the electrolyte, each cell having its own circulation system, contact roll and anode bank. The process involves passing the steel strip horizontally across the upper surface of the electrolyte in a series of the cells so that the strip is plated only on the bottom side. This is followed by passing the strip upwardly and backwardly so that the original top of the strip becomes the bottom, and then passed across a further series of plating cells so that this bottom side also becomes electrolytically plated with tin. Halogen-type lines have the advantage of high strip speed operation and further, different coating weights can be applied to the opposite faces of the strip.
Typical baths comprise aqueous solutions of stannous tin chloride and fluoride ions as well as ferrocyanide ions to precipitate any ferric ion formed in the bath as a result of its contact with the steel substrate. Typical electrolyte solutions contain the following compositions:
1. Stannous Ions (Sn II) 12 to 25 grams per liter; PA1 2. Chloride Ions 38 grams per liter; PA1 3. Fluoride Ions 34 grams per liter; and PA1 4. Ferrocyanide Ions 0.75 grams per liter. PA1 1. stannous chloride 75 g/l; PA1 2. sodium fluoride 30 g/l; PA1 3. sodium bifluoride 45 g/l; PA1 4. sodium chloride 50 g/l; and PA1 5. pH 3.2-3.6. PA1 (a) a stannous tin halide; and PA1 (b) a salt having PA1 (a) a stannous tin halide; PA1 (b) a ferric iron salt; PA1 (c) a salt having
The above materials may be varied anywhere from about .+-.10% to about .+-.40% and especially from about .+-.15% to about .+-.30%.
The coated strip is then rinsed in a fluoride ion containing rinsing solution such as an aqueous solution of sodium bifluoride and/or sodium fluoride. The rinsing solution preferably has a pH below about 4. Coating thicknesses anywhere from about 0.5 to about 1.5 g/m.sup.2 are typically applied in this process.
Rogers, et al., U.S. Pat. No. 3,920,524, describes a similar process and particularly note that the substrate is passed through the electroplating solution at a rate of from about 90 to about 1,000 meters per minute where the potential applied is adjusted preferably from about 5 to about 25 volts with a current density being maintained at from about 0.2 to about 30 kiloamperes per square meter. Typical electroplating bath solution temperatures vary from about 45.degree. C. to about 50.degree. C.
Rogers, et al., further describe recirculating the electrolyte while moving the steel substrate through the electrolyte.
In one example, Rogers et al. describe the electrolytic deposition of tin onto a 100 cm wide carbon steel strip in a 1.5 meter deep tank using platinum-clad tantalum anodes. The example teaches circulating the electroplating solution in a 1.5 meter deep tank slightly wider than 100 cm at a rate of about 1135 liters per minute with the steel substrate travelling at a speed of about 90 to about 1000 meters per minute so as to vary the thickness of the electrodeposits from about 0.75 to about 3.0 micrometers. The electrolyte is maintained at a temperature of from about 45.degree. to about 50.degree. C. by appropriate heat exchange devices.
Application of a 20 volt potential across the assembly in the work piece provides a current density on the anode of about 4 kiloamperes per square decimeter to achieve a cathode current efficiency of from about 90 to about 97%.
Nobel, et al., U.S. Pat. No. 5,094,726, describes a similar halogen-tin electroplating process employing jet agitation or vigorous solution movement. Nobel, et al. specifically note that the industry achieves high speed plating by the use of high current densities and particularly high cathode efficiencies through the use of vigorous agitation and elevated solution temperatures.
Utilizing high speed agitation with the resultant rapid pumping action of the electrolyte and solution movement results in air mixed with the electrolyte promoting oxidation of Sn(II) to Sn(IV) and Fe(II) to Fe(III) where iron is pulled into the bath by the action of the electrolyte on the steel substrate. Both of these elements result in the production of sludge that reduces the efficiency of the bath and clogs or plugs the jets and spargers of the agitation system resulting in frequent and costly production shutdowns for cleanup and sludge removal. Sludge, however, can be minimized to some degree by reducing agents such as pyrocatechol, resorcinol, or hydroquinone. Nobel, et al. employs various imidazolines to minimize sludge formation.
The related art describes various methods of sludge removal, such as Fisher, et al., U.S. Pat. No. 4,006,213, describing methods for recovering hydrated stannic oxide and alkaline metal ferrocyanide whereas Thompson, et al., U.S. Pat. No. 5,378,347, incorporates various antioxidants into the halogen tin bath, such as a Group IV B, V B, or VI B elements from the periodic table of elements.
Typical tin baths employed by Thompson, et al. include:
Although not stated by Thompson et al, it is typical in the art to vary the composition of the foregoing bath anywhere from .+-. about 10% to .+-. about 40%, especially .+-. about 15% to .+-. about 30%.
Beale, U.S. Pat. No. 3,623,962, minimizes sludge formation by the continuous deaeration of a halogen-tin electrolyte to remove gases absorbed when the electrolyte is exposed to ambient atmosphere, thereby decreasing the opportunity of the electrolyte to absorb oxygen.
Stuart, et al., U.S. Pat. No. 4,219,390, describes a method for regenerating an electrolytic tinning bath in which the bath is freed from ions of foreign metal introduced during tinning, by detinning the bath electrolytically and removing the foreign metal ions by means of a cation exchanger.
Horn, U.S. Pat. No. 3,907,653, treats the sludge of a halogen tin plating bath containing both sodium fluorostannate and iron ferrocyanide by forming various solutions and complexes followed by precipitating the various components.
Swalheim, U.S. Pat. No. 2,372,032, notes that ordinarily the removal of fluorostannate sludge presents no difficulty when settled out or filtered out of the plating bath, but the recovery of the tin content of the sodium fluorostannate bath presented a difficult problem. Swalheim describes treating a halogen-tin plating bath sludge by converting an alkali fluorostannate to stannous fluoride and an alkali fluoride by effecting contact of the fluorostannate with molten tin, preferably in the presence of residual stannous fluoride.